All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible to pay Zakah to the parents-in-law if they are Muslims and are eligible to receive Zakah.
For more benefit on the people who are entitled to be given Zakah, please refer to Fataawa 88368 and 86546.
Allaah Knows best.